1. Field of the Invention
The present invention relates to an image forming apparatus such as a laser printer.
2. Description of Related Art
A conventional image forming device, such as a laser printer, includes a photosensitive drum. A charge unit, a scanner, a developing roller, and a transfer roller are provided in this order around the photosensitive drum following the rotational direction of the photosensitive drum. As the photosensitive drum rotates, the following sequences of image forming processes are executed. That is, first, the surface of the photosensitive drum is entirely charged by the charge unit. Then, the surface of the photosensitive drum is exposed by a high speed scan of a laser beam from the scanner that is modulated by image data. As a result, an electrostatic latent image is formed on the photosensitive drum based on the image data. Next, as a result of rotation of the developing roller, toner borne on the surface of the developing roller is brought into contact with the photosensitive drum, and the toner on the developing roller is supplied to the electrostatic latent image on the photosensitive drum. As a result, toner is selectively borne on the photosensitive drum, thereby forming a visible toner image. Afterwards, the visible toner image borne on the surface of the photosensitive drum is transferred onto a sheet when the sheet passes between the photosensitive drum and the transfer roller.
Normally, in the image forming device of this type, the transfer roller is applied with a transfer bias in order to transfer the visible toner image onto a sheet. According to one conventional method, a transfer bias voltage of a fixed value is applied to the transfer roller. In this case, the value of the transfer bias voltage is controlled to be maintained to the fixed value by executing a constant voltage control operation. According to another conventional method, a transfer bias current of a fixed amount is applied to the transfer roller. In this case, the amount of the transfer bias current is controlled to be maintained to the fixed amount by executing a constant current control operation.
When a control is executed to apply a transfer bias voltage of a fixed value to the transfer roller, if the environmental humidity changes, the resistance value of the transfer roller also changes. This induces change in the amount of the current flowing through the transfer roller. For example, when the resistance value of the transfer roller increases to a too great value, the amount of the transfer current decreases to a too small amount. This will cause transfer problems.
On the other hand, when a control is executed to apply a transfer bias current of a fixed amount to the transfer roller, even if the environmental humidity changes and therefore the resistance value of the transfer roller changes, the fixed amount of current continues flowing through the transfer roller. Accordingly, it is desirable to apply the transfer bias current of a fixed amount to the transfer roller by executing the constant current control.
When the control is executed to apply the transfer bias current of a fixed amount to the transfer roller, however, if a sheet whose width is narrower than that of the transfer roller, is transported to the transfer roller, no part of the sheet exists between the photosensitive drum and the transfer roller at their axial ends. Accordingly, the transfer roller and the photosensitive drum directly contact with each other at their axial ends. A large amount of transfer current flows into the photosensitive drum directly from the transfer roller at those areas where the transfer roller directly contacts the photosensitive drum. This will decrease the amount of the transfer current that can appropriately flow through the sheet to transfer a visible toner image onto the sheet. This will cause transfer problems.
In order to solve this problem, Japanese Patent Application Publication No. 2-272590 has proposed to change the amount of the transfer bias current in accordance with change in the width of a sheet.
Even if the transfer bias current amount is changed dependently on the width of a sheet, however, transfer problems will still occur when the resistance value of the transfer roller changes according to change in the environment humidity.
For example, if the environment humidity becomes extremely high, the resistance value of the transfer roller will decrease. Accordingly, the proportion of the transfer current that flows into the photosensitive drum directly from the transfer roller will increase, and the amount of the transfer current that flows through the sheet will decrease. This will generate transfer problems. It is therefore conceivable to set the amount of the transfer current as being fixed to a large value in order to prevent this problem from occurring even if the environment humidity increases. However, in such a case, if the environmental humidity decreases, the resistance value of the transfer roller will increase, and the voltage generated through the transfer roller will increase. This will generate an electric discharge, and will occur transfer problems.
In view of the above-described drawbacks, it is an objective of the present invention to provide an improved image forming apparatus that is capable of attaining an appropriate transfer operation by executing a constant current control even when the type of recording media or the environmental humidity changes.
In order to attain the above and other objects, the present invention provides an image forming apparatus, comprising: an image bearing unit bearing a developing agent image thereon; a transfer unit transferring, at a transfer position, the developing agent image from the image bearing unit to a recording medium; a bias applying unit capable of applying a transfer bias current to the transfer unit while maintaining fixed the amount of the transfer bias current; a measuring unit measuring a resistance of the transfer unit; a type detecting unit detecting a type of the recording medium; and a control unit determining the amount of the transfer bias current, based on the detected recording medium type and on the measured resistance value, the control unit controlling the bias applying unit to apply the determined transfer bias current to the transfer unit.